1. Field of the Invention
The present invention relates to new supported metallocene-alumoxane catalysts. More particularly, the present invention relates to the production of polyolefins, particularly of high density polyethylene homopolymers or copolymers, having a broad monomodal molecular weight distribution wherein the polymerization process is conducted in the presence of the new supported metallocene-alumoxane catalysts.
2. Description of the Prior Art
For polyolefins in general and high density polyethylene in particular, hereinafter referred to as polyethylene, the molecular weight distribution (MWD) is one of the basic properties that determines the properties of the polymer, and thus its end-uses.
Although it may be difficult to evaluate the influence of each property taken independently, it is generally accepted that the molecular weight mostly determines the mechanical properties while the molecular weight dispersion mostly determines the rheological properties.
There is a demand for high molecular weight polyethylene, because an increase of the molecular weight normally improves the physical properties of the resins. However, high molecular weights tend to make polymers harder to process. On the other hand, an increase in the MWD tends to improve the flowability at high shear rate during the processing. Thus, broadening the MWD is one way to improve the processing of high molecular weight (=low melt flow index) polyethylene, in applications requiring fast processing at fairly high die swell, such as in blowing and extrusion techniques.
It is generally believed that, in polyethylene having a high molecular weight combined with a broad MWD, the lower molecular weight portion aids in processing while the higher molecular weight portion contributes to the good impact resistance of the film, such polyethylene being processed at higher throughput rates with lower energy requirements.
The MWD may be described completely by the curve obtained by gel permeation chromatography. The MWD is generally described by a figure which is a good evaluation, also called the polydispersity index, representing the ratio of the weight average to the number average molecular weight.
There are several known methods of producing polyethylene having a broad and multimodal MWD; however, each method has its own disadvantages. Polyethylene having a multimodal MWD can be made by employing two distinct and separate catalysts in the same reactor each producing a polyethylene having a different MWD; however, catalyst feed rate is difficult to control and the polymer particles produced are not uniform in size and density, thus, segregation of the polymer during storage and transfer can produce non-homogeneous products. A polyethylene having a bimodal MWD can also be made by sequential polymerization in two separate reactors or blending polymers of different MWD during processing; however, both of these methods increase capital cost.
European Patent No 0128045 discloses a method of producing polyethylene having a broad molecular weight distribution and/or a multimodal MWD. The polyethylenes are obtained directly from a single polymerization process in the presence of a catalyst system comprising two or more metallocenes each having different propagation and termination rate constants, and aluminoxane.
It is interesting to note that the known methods of preparing broad molecular weight distribution polyolefins show a bimodal or multimodal MWD. Indeed, the gel permeation chromatograph curves show a more or less marked bimodal or multimodal MWD of the polyolefin. The MWD and shear rate ratios of the polymer and the catalyst activity disclosed in the known methods are rather low. Further the known metallocene catalyst systems for producing broad MWD use aluminoxane as cocatalyst during the polymerization which is not suitable for the slurry, bulk and gas phase processes and which causes severe fouling inside the reactor and renders the use of such a type of catalyst in continuous processes almost impossible.